相同的表达式，但平均在循环中花费的时间要少得多

#include <iostream>
#include <chrono>

int main() {
    const int N = 1000000;
    int sum = 0;

    // 不优化的版本
    auto start = std::chrono::high_resolution_clock::now();
    for (int i = 0; i < N; ++i) {
        sum += i * i;
    }
    auto end = std::chrono::high_resolution_clock::now();
    std::chrono::duration<double> elapsed = end - start;
    std::cout << "不优化的版本耗时: " << elapsed.count() << " 秒\n";

    // 优化的版本
    start = std::chrono::high_resolution_clock::now();
    int square = 0;
    for (int i = 0; i < N; ++i) {
        square = i * i;
        sum += square;
    }
    end = std::chrono::high_resolution_clock::now();
    elapsed = end - start;
    std::cout << "优化的版本耗时: " << elapsed.count() << " 秒\n";

    return 0;
}